1. Field of the Invention
The present invention relates to a method of cold drawing seamless metal tubes each having an upset portion on each end.
These seamless metal tubes are preferably used for drilling tubes, casings, inner rods and outer rods for geological, mineralogical, metallurgical or geothermal research and/or drilling for civil engineering or water wells.
2. Prior Art
With reference to FIGS. 1 to 4(b) showing the seamless metal tubes which have been produced by the conventional method using the conventional draw bench, a fot forged thin wall midbody 7c of a metal tube is welded at each end with a thick wall end tube 7b in order to obtain a large diameter seamless metal tube or pipe 7 having an upset portion 7b on each end.
Alternatively, the seamless metal tube may be produced by casting or lathing, but metallic filaments in such a seamless metal tube are axially cut at welded or lathed portions or at blowholes, which are likely to cause cracks in the seamless metal tube after heat treatment at high temperature.
Thus, a method of and a draw bench for cold drawing seamless metal tubes each having an upset portion on each end have been proposed. For example, a small diameter and thin wall seamless metal tube having a diameter of 25-35 mm and a wall thickness of 2.1-3.2 mm, such as for a bicycle framework, may be cold drawn through the draw bench shown in FIGS. 2(a) to 4(b).
The conventional draw bench, which is particularly shown in FIGS. 2(a) and 2(b), for drawing such seamless metal tube comprises a bed 12 to be fixed on the floor, a die holder 26 which is mounted at a middle portion on the bed 12, and a closed periphery reducing die 2 which is concentrically mounted in the die holder 26. A pair of tongs 6 and a bearing are arranged on the entry side of the bed 12 for horizontally supporting a plug fixing rod 88.
Secured to a front portion of the rod 88 is a plug 4 which is inserted into the reducing die 2 when drawing.
A draw unit 110 is arranged on the exit side of the bed 12 and comprises a carriage 112 mounted on rollers travelling on the bed 12, the carriage 112 carrying a hook 118 and a chuck 114 for gripping the front tip 7a of the mother tube or workpiece 7. The carriage 112 is driven by an endless chain 120 encompassing a driven sprocket wheel (not shown) mounted on the bed 12. A driving sprocket wheel is coupled by any convenient means with an electric motor (not shown). The hook 118 is engaged with the endless chain 120 when drawing, and the front tip 7a of the mother tube 7 engaged by jaws 116 of the chuck 114 is strongly pulled by the carriage 122 to draw a tube 7 having an upset portion 7b on each end.
The embodiments in FIGS. 3(a) and 3(b) show the conventional manner of drawing seamless metal tubes which have the same outer diameter but unequal inner diameter upset portions on both ends. In FIG. 3(a), a plug 4 is held adjacent to a tapered entry in the axial bore of the reducing die 2, at a clearance from the inner periphery of the mother tube 7, to draw the portion of the tube 7 having a small inner diameter upset portion 7b on both ends, while the plug 4 in FIG. 3(b) is held right in the bore of the reducing die 2 to draw a tube 7 having a larger inner diameter.
FIGS. 4(a) and 4(b) show another conventional drawing method.
As shown in FIG. 4(a), a reducing die 2 can have an internal surface including an inlet portion 2b which tapers forwardly toward a bearing or throat portion 2a, and oppositely inclined outlet portion 2c. When the enlarged front portion 4a of a plug 4 is held right in the bearing portion 2a of the reducing die 2, the mother tube 7 is drawn through the bearing portion 2a and over the enlarged front portion 4a to form a drawn tube having a small outer diameter.
In FIG. 4(b), the enlarged front portion 4a of the plug 4 is passed leftwards through the bearing portion 2a and located adjacent to the outlet portion 2c. Then the mother tube 7 is drawn through the bearing portion 2a and around the small diameter portion 4b of the plug 4 to reduce the outer diameter of the drawn tube. When the mother tube 7 is further advanced, the inner diameter of the mother tube 7 is widened by the enlarged front portion 4a of the plug 4 so as to draw a tube 7 having the same inner diameter as that of the enlarged front portion 4a of the plug 4 and having an outer diameter upset portion 7b larger than that of the drawn tube 7 produced by means of the reducing die 2 of FIG. 4(a).
Seamless metal tubes 7 drawn through conventional reducing dies 2 as described above have the following disadvantages:
(1) The seamless mother tube 7 having a small inner diameter is drawn through the reducing die 2 of FIG. 3(a) without any internal radial pressure, thus resulting in corrugation in and around the drawn tube.
(2) In FIG. 4(b), the mother tube 7 is drawn through the reducing die 2 with the plug 4 held in the bearing portion 2a and in the tube 7 to obtain the drawn tube 7 having one outer diameter but two different inner diameter upset portions 7b. The tube drawn through the bearing portion 2a and over and around the small diameter portion 4a of the plug 4 is widened by the enlarged front portion 4a of the plug 4 to obtain a drawn tube similar to that shown in FIGS. 7(a), 7(b) and 7(c), wherein
d.sub.4 denotes a large diameter bearing portion of the plug 4, d.sub.2 denotes a bearing portion diameter of the reducing die 2, d.sub.1 denotes a large diameter of the drawn tube 7, and d.sub.3 denotes a small diameter bearing portion of the plug 4 respectively. PA1 (a) one having one outer diameter but two unequal inner diameters; PA1 (b) another having one inner diameter but two unequal outer diameters.
Accordingly, d.sub.4 becomes an inner diameter of the drawn tube 7 and d.sub.2 becomes a small outer diameter of the drawn tube 7, but the large diameter d.sub.1 of the drawn tube 7 is not directly related to the bearing portion diameters of the reducing die 2 and the plug 4, but is given by the following functional formula: EQU d.sub.1 =f(d.sub.2, d.sub.3, d.sub.4)
However, we cannot determine the values of d.sub.1, d.sub.2 and d.sub.4 independently. In order to arrive at the most preferable values for d.sub.1, d.sub.2 and d.sub.4, it is necessary to select the values sufficiently near the most suitable value among the various solutions of the functional formula of EQU d.sub.1 =f(d.sub.2, d.sub.3, d.sub.4)
We cannot, however, obtain the most suitable values for the diamteers of d.sub.1, d.sub.2 and d.sub.4.
(3) The configuration of the drawn tube is limited to only two kinds, i.e.
(4) The drawing force of the plug 4 is so small that the plug 4 may be driven in its movement by a hydraulic cylinder, while on the other hand it is necessary to provide a balancing unit or a plurality of hydraulic cylinders to balance the reaction upon the strong drawing force of the plug 4, thus making the device complicated and expensive.